1. Field of the Invention
The present invention relates to an electronic ballast for gas discharge lamps that provides for regulation of lamp run-up current.
2. Description of the Related Art
Typically, immediately after ignition, arc discharge lamps, such as HID lamps, go through a run-up phase where the lamp heats up and the pressure in the discharge builds. At the beginning of run-up the lamp voltage is typically low (20V). It is usually desirable to provide a current to the lamp during run-up that is greater than the steady state current in order to properly heat the electrodes and to build pressure in the lamp more quickly. As the lamp progresses through run-up, the lamp voltage will eventually reach its steady state value (e.g. 90V) and the circuit should be delivering the proper steady state current.
In the present state of the art, regardless of the circuit topology used to drive the lamps, a means of properly controlling the run-up current is required. This typically requires a means of sensing the lamp current and the lamp voltage with suitable controls to regulate the run-up current in response to the lamp conditions. Sensing the lamp current and voltage can contribute to losses in the circuit.
Thus, what is also needed is a method and corresponding circuit that effects control of the run-up current properly in HID lamps without requiring sensing the lamp current or lamp voltage.
The present invention is directed to an electronic ballast with lamp run-up current regulation. In one aspect, the electronic ballast comprises an input stage coupled to an AC source, the input stage converting an AC voltage to a direct current bus voltage, an output stage having inputs coupled to the bus voltage and outputs connected to a lamp, the output stage providing (i) power to the lamp so as to produce a lamp voltage and lamp current in a steady state mode of operation, and (ii) a lamp run-up current to the lamp during a run-up phase of the operation of the lamp, and a current regulation circuit for regulating the lamp run-up current so that the lamp run-up current exceeds a steady state lamp current value, and increases if either the bus voltage increases or the lamp voltage decreases. The output circuit comprises a main high-frequency switching inductor through which an inductor current flows wherein the lamp current is based upon the inductor current. In one embodiment, the current regulating circuitry further includes current limiting circuitry for limiting the lamp run-up-current to a predetermined value. In another embodiment, the current regulating circuitry further comprises a feedback circuit that adjusts the magnitude of the lamp run-up-current in accordance with the magnitude of the bus voltage. In a further embodiment, the current regulating circuit further comprises circuitry for limiting the magnitude of the inductor current at the moment of commutation.
In a related aspect, the present invention is directed to a method for operating an electronic ballast comprising the steps of (a) providing an electronic ballast comprising an input stage coupled to an AC voltage source and including circuitry for converting an AC voltage to a direct current bus voltage, an output stage having inputs coupled to the bus voltage and outputs connected to a lamp wherein the output stage provides power to the lamp so as to produce a lamp voltage and lamp current, and a current regulation circuit for regulating the lamp run-up current, the electronic ballast having an ignition mode of operation, a post-ignition mode of operation immediately subsequent to the ignition mode of operation wherein the ballast provides a run-up current to the lamp, and a steady state mode of operation, (b) initiating the ignition mode of operation of the electronic ballast, (c)thereafter, initiating the post-ignition mode of operation, and (d) thereafter regulating the lamp run-up current so that the lamp run-up current exceeds a steady state lamp current value, and so that the lamp run-up current increases if either the bus voltage increases or the lamp voltage decreases.
In one embodiment, the method includes the step of limiting circuitry for limiting the lamp run-up-current to a predetermined value.
In another embodiment, the method includes the step of adjusting the magnitude of the lamp run-up-current in accordance with the magnitude of the bus voltage.
In a further embodiment, the method includes the step of limiting the magnitude of the inductor current at the moment of commutation.